The use of sound signals (i.e. structure-borne sound signals generated by at least one electric arc, which are propagated through the melt stock, or airborne sound signals which are propagated through the air volume between the melt stock) to generate various characteristic values is known. In this case sound vibrations are recorded, which can be evaluated by taking into account the current and voltage profiles of the electric arcs of the arc furnace. Sound signals are inherently created in the interior of the furnace fill, since the electric arcs of the arc furnace constitute a sound source.
According to DE 10 2008 006 965 A1, it is known for example that in order to determine a so-called radiation measure (also abbreviated to SM below), structure-borne sound vibrations on the furnace wall are recorded and an associated vibration evaluation signal can be determined from a frequency range of the recorded vibrations. From the recorded electrode current, an associated current evaluation signal can be determined in the same frequency range, which is interpreted as a cause of the vibration generation. The radiation measure is then given in principle as the ratio of the vibration evaluation signal and the current evaluation signal.
According to DE 10 2008 006 966 A1, it is furthermore known that a so-called lumpiness measure (also abbreviated to M below) can be determined by recording the supplied electrode current, determining an rms value measure from the recorded electrode current and furthermore determining an associated current component from the recorded electrode current in a particular frequency range of the recorded electrode current. The lumpiness measure is then given as the ratio of the current component and the rms value measure.
Furthermore, it is known from DE 10 2008 006 958 A1 that a so-called measure of the change in the mass of a melt stock component located on the boundary of the arc furnace (also abbreviated to MM below) can be determined by recording the supplied electrode current, from which a current evaluation signal is obtained in a particular frequency range. The structure-borne sound vibrations are furthermore recorded and a vibration evaluation signal is determined in the particular frequency range. Lastly, the phase shift between the current evaluation signal and the vibration evaluation signal is determined for a multiplicity of common frequencies. From these phase shifts which have been determined, a measure of the change in the mass of the melt stock located on the boundary of the furnace wall can be derived.
With the aforementioned characteristic values, a refined method for controlling the melt process in the arc furnace can be carried out. In order to illustrate this, the melt process taking place in arc furnaces will be explained in more detail below. An arc furnace is used to produce liquid metal, generally steel. The liquid metal is produced from solid melt stock, for instance scrap and/or reduced iron (so-called sponge iron or DRI/HBI) or else with liquid and/or solid pig iron, together with further additives. To this end, energy for melting the melt stock may be introduced into the arc furnace by means of three electrodes, generally in the form of an electric arc between an electrode and the melt stock. So that the melting can take place as efficiently as possible, as far as possible all the energy provided by the electric arc may be introduced into the melt stock. The melt stock is in this case intended to mean the solid to be melted, and molten material is intended to mean liquid metal and/or slag. Melt stock and molten material together make up the furnace fill.
Owing to the predetermined operating procedure in conventional arc furnaces, however, the electric arc may burn free during the melt-in process. This means that the thermal radiation emitted by the electric arc formed between the electrode and the melt stock to a large extent reaches a boundary of the arc furnace, in particular a cooled wall of the arc furnace. This increases the energy consumption of the furnace, on the one hand because the energy of the arc furnace is introduced into the melt stock only to a relatively small extent, and on the other hand more energy is dissipated via the furnace cooling system.
In this context, the idea arises to use the measure MM of the change in the melt stock located on the furnace wall, the lumpiness measure M, the radiation measure SM or similarly suitable characteristic values for the distribution of melt stock, melt and slag in the furnace fill, in order to control the operating procedure of arc furnaces and regulate the electric arc power. The lumpiness measure M may be used in order to regulate the electrode current setpoint value for the electrodes. If, for example, there is comparatively light scrap below an electrode, i.e., a high proportion of air volume in the scrap, then the radiant power may be stepped down in order to prevent the aforementioned free burning of the electric arc due to excessively rapid melting of the light scrap. If an excessively high radiation measure SM is identified on the furnace walls, then the radiant power of the electric arc may be stepped down in order to avoid excessive thermal loading of the furnace walls and a high power loss. If, when determining the shielding measure SM, it is found that a part of the furnace wall is not shielded by melt stock, the radiant power may be stepped down in order to prevent free burning of the electric arc into this free wall section. In this context, the aforementioned signals may be used not only for power reduction but also, in the inverse interpretation, also for power increase. However, since the measures indicated above influence one another, in the case of manual intervention in the running program of the arc furnace it is difficult to estimate how much to intervene in the process.